Development of a new bio-impedance sensor to detect retinal vessel punctures for retinal vein occlusion treatment

Retinal vein occlusion is a blockage of a retinal vessel by a clot, preventing a proper circulation of oxygenated blood through the eye. This disease is estimated to affect up to 16.4 million persons worldwide (Rogers, 2010). A curative treatment is still under research. A promising approach, known as retinal vein cannulation, consists of dissolving the clot by injecting a thrombolytic agent directly inside the affected vessel during a few minutes (Van Overdam, 2015). However, such gesture is complex to perform and presents a great risk for complications due to a combination of limited depth perception, the fragility of the vessels (size from 50 to 400 μm, puncture force below 20 mN) and the disturbance caused by physiological hand tremor (typical RMS amplitude around 180 μm). Robotic assistance techniques that can stabilize the tool and downscale the surgeon's force or motion have been under study quite some time now. Recently, a first inhuman robot-assisted cannulation has been successfully performed. However, whereas the robotic assistance already helps in conducting a controlled puncture, determining the exact instant that puncture takes place remains difficult. By introducing sensorised instruments this part could be simplified and double puncture prevented more easily (Gijbels, 2015). In this paper, a new sensitive instrument based on bioimpedance measurement is proposed. Its limited cost and simplicity make it a very promising tool. Bio-impedance sensors have been developed earlier for minimally invasive surgery. Since each tissue layer has a specific impedance, the entry into a new layer could be detected as a change in impedance. Bio-impedance instruments are generally composed of two conductive parts insulated from each other. The biological tissue ensures typically the closure of the electrical path at the instrument tip. Several successful demonstrations of this measurement principle have been reported e.g. in animal, to detect prostate cancer (Lee, 1999), access to the renal collecting system (Hernandez, 2001) or vessel puncture (Saito, 2006) or in human, to detect the spinal fluid (Halonen, 2016). In the following we introduce a new bio-impedance sensor that was developed to detect the puncture of retinal vessels. A prototype sensor has been integrated into a cannulation needle and measurements have been done showing impedance variations while cannulating retinal vessels of ex-vivo pig eyes.